Exp 3 Diffusion Lab Handout

Exp 3 Diffusion Lab Handout - Department of Chemical...

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Department of Chemical Engineering and Materials Science University of California, Davis ECH 155A Chemical Engineering Laboratory Stefan Diffusion Tube Experiment Goals of the Experiment Primary goal of this experiment is to measure diffusion coefficients of vapors of three separate compounds by following the displacement of a liquid-air interface. Theoretical Background Stefan (1873) first developed this method. Assuming that diffusion occurs at quasi-stationary state, and that the concentration is zero at the top of the diffusion tube (Fig. 1b), it can be shown that the diffusivity is related to the position of the interface and the diffusion time by the following equation (Lee and Wilke, 1954): ( 29 ( 29 ( 29 D = ln 2 2 2 o o e t t l x λ (1) One implicit assumption underlying Eq. (1) is that there is no turbulence at the top of the diffusion tube. Some experiments have shown this to be true for Re DT < 20. When the diffusivity, D , is measured by following the meniscus with a cathetometer, statistical analysis of the data can be simplified by rearranging Eq. (1) to the following form: 2 2 2 1 = + ( – )ln( – ) D t t x o e o λ (2) Thus, D may be estimated from the constant that multiplies the time variable, (t – t o ), in this linear relationship. See the Supplement for more details. Experimental Apparatus The apparatus (Fig. 1a) consists of a brass tube to which diffusion tubes are attached; the connection between the diffusion tubes and the brass section is made through “leak-proof” fittings with Teflon ferrules; a thermostat bath; a rotameter; a cathetometer; thermometers; and a tube filled with desiccant to dehumidify the inlet air. The initial level of the volatile liquid in the diffusion tube should not be very near the top of the tube (why not?). A distance of 4 cm from the top may be a good one to avoid very lengthy runs. Experimental Procedure AM Groups will setup the experiment and take measurements until their laboratory time is over. All data is to be neatly tabulated for the continuing PM Group. PM Groups will continue the AM Group experiments and clean up the equipment at the end of their laboratory time. PM Groups will provide their data neatly tabulated to their partner AM Group. Procedure: Make measurements for acetone and the other two compounds at two temperatures.
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Exp 3 Diffusion Lab Handout - Department of Chemical...

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